If this is your first visit, be sure to
check out the FAQ by clicking the
link above. You may have to register
before you can post: click the register link above to proceed. To start viewing messages,
select the forum that you want to visit from the selection below.
Please be aware that this is a community forum. Although Holley employees will take part in answering questions from time to time, your questions may or may not be answered by Holley personnel. Thus, Holley can not guarantee an answer or timing on an answer.

TBI-to-MPI Ported Throttle Body

This is my 900 CFM TBI throttle body that I converted to multi-port use (by removing the injector pod) and ported for better airflow. Four barrel, progressive linkage throttle bodies are excellent because they provide a good linear increase in airflow. I also wanted to improve the airflow so it would be comparable to the 1000 CFM MPI throttle bodies. They already have the same size throttle blades/bores but I knew the airflow could be improved.

Through correspondence with Doug F. @ Holley, I learned the TBI units could flow 80-100 CFM more air, if the bore dividers (walls) were raised to the top and the ridges (midway) ported smooth. Evidently, Holley engineers have flow tested and proven this (it's effective even with the injector pod installed). As you can see, the original TBI bores are machined flat & low, which hinders the airflow. So I completely disassembled the TBI unit and mig welded (4043 aluminum .035" wire) material up to the top. Then I ported the bores with a carbide bur inserted in a drill press. Afterwards, I finished porting the bores with a wire wheel. I ported the IAC passage too. The result shown below. (The last two pictures are for reference only.)

TIP: Holley did testing with a 1" open spacer under their 900 CFM TBI unit, and it actually improved the air/fuel distribution and made 10-15 more horsepower above 4800 RPM.

At that time, I reassembled the TBI unit & injector pod, and it ran great on my engine. (I replaced the 1¾" throttle blades with Holley #26-97.) Since then, I converted my Edelbrock Performer RPM 'Air-Gap' dual-plane intake manifold to multi-port EFI. I purchased all the required components, and my throttle body was ready for the conversion.

Update:
Soon afterwards, I decided to experiment with a 1:1 ratio secondary throttle linkage. I'm glad I did. The 1:1 ratio
linkage is supposed to have better part throttle air distribution and feel more responsive. The 1:1 ratio linkage is
an improvement! I think large cubic inch engines can tolerate the immediate additional airflow without problemsand may actually need it. Also, I think heavy weight vehicles dampen the effect to a point where progressive
linkages feel unresponsive and lazy until you near ½ throttle.

After determining that I liked the feel, I custom made my own adjustable link and installed it. I machined a pin to
fit the smaller hole in the primary lever, then threaded it for the #8-32 bolt I bent to fit the secondary lever. The
throttle body levers are not modified in any way. The link was made to fit the throttle body as is. I think Holley20-122 may be easily adapted for this use, because a longer piece of threaded rod could lengthen the linkage
(if necessary):

Notes:
The 700 cfm TBI throttle body has 1½" bores, and actually flows about 650 cfm with the injector pod.
The 900 cfm TBI throttle body has 1¾" bores, and actually flows about 850 cfm with the injector pod.
The 950 cfm TBI throttle body has 1¾" bores, and actually flows 950 cfm because there's no injector pod.
Holley TBI units are CFM rated the same way as four-barrel carburetors - at a depression of 1.5 in/Hg.
(Two-barrel carburetors are rated at a depression of 3.0 in/Hg.)

Fuel injected engines don't suffer from oversize throttle bodies like carbureted engines do,
because the fuel can always be properly metered to the amount of air entering the engine.
Fuel injected engines don't rely on engine breathing (venturi effect) for proper fuel metering.

FYI:
Be careful when shopping for a throttle body. Some manufactures advertise skewed (higher) CFM ratings.
Basically, if it sounds too good to be true, it probably is. The following quote is a good example:

Originally Posted by Doug F. @ Holley

I'd be a little leery of that design... FWIW, Accel sells a "1200 CFM" throttle body. Holley's is a "1000 CFM".
Accel must use some odd-ball depression because when I flowed theirs at 4BBL depression (1.5"), it flowed below 900 I believe.
It has smaller plates so... Point is, four 1.75" plates (Holley) with no boosters is pretty big.

Posting Permissions

About us

Holley has been the undisputed leader in fuel systems for over 100 years. Holley carburetors have powered every NASCAR® Sprint® Cup team and nearly every NHRA® Pro–Stock champion for four decades. Now, Holley EFI is dominating the performance world as well as our products for GM's LS engine. Holley's products also include performance fuel pumps, intake manifolds & engine dress–up products for street performance, race and marine applications. As a single solution, or partnered with products from other Holley companies - Hooker Headers, Flowtech Headers, NOS Nitrous, Weiand, Earl's Performance Plumbing, or Diablosport - Holley products can give you the edge you need over the competition.